The findings showed that every five per cent of dietary energy that was switched from carbohydrates or saturated fats to mono- or poly-unsaturated fats could lead to a 22 per cent reduction in the risk of type 2 diabetes.

Substituting roughly 100 calories of unsaturated fats, especially polyunsaturated fats, for 100 calories of carbohydrate or saturated fat also lowered the risk of cardiovascular diseases by 6.8 per cent.

The study was led by Dr Dariush Mozaffarian, dean of the Friedman School of Nutrition Science and Policy at Tufts University and Dr Fumiaki Imamura, from the Medical Research Council (MRC) Epidemiology Unit at the University of Cambridge.

Mozaffarian said: "The world faces an epidemic of insulin resistance and diabetes. Our findings support preventing and treating these diseases by eating more fat-rich foods like walnuts, sunflower seeds, soybeans, flax seed, fish, and other vegetable oils and spreads, in place of refined grains, starches, sugars, and animal fats.

"This is a positive message for the public. Don't fear healthy fats."

It is hoped the study findings could now be used to help educate doctors and patients about the importance of diet on health and the risk for type 2 diabetes.

Dr Imamura added: "Until now, our understanding of how dietary fats and carbohydrates influence glucose, insulin and related risk factors has been based on individual studies with inconsistent findings.

"By combining results from more than 100 trials, we provide the strongest evidence to date on how major nutrients alter these risks."